Christine Blasey Ford has agreed to testify before a US Senate panel next week over allegations of sexual assault.

The woman whose sexual assault allegation threatens to bring down President Donald Trump's Supreme Court nominee has agreed to testify in the Senate, her lawyers said Saturday according to US media.

Christine Blasey Ford's decision came after days of negotiations and following President Donald Trump's turn against her, saying her accusation could not be true.

Ford "accepts the Committee's request to provide her first-hand knowledge of Brett Kavanaugh's sexual misconduct next week," said a message from her lawyers to the Senate Judiciary Committee, US media reported.

The committee had given Ford, a California professor, until 2:30pm (18:30 GMT) Saturday to decide on whether to appear, after she rejected a Friday evening deadline imposed by the committee's Republican leader, Chuck Grassley.

"Although many aspects of the proposal you provided via email, on (Friday) are fundamentally inconsistent with the committee's promise of a fair, impartial investigation into her allegations, and we are disappointed with the leaks and the bullying that have tainted the process, we are hopeful that we can reach agreement on details," the lawyers' letter cited by The Washington Post said.

Ford alleges that Brett Kavanaugh drunkenly assaulted her in the 1980s at a party when he was 17 and she was 15, they were attending private schools outside Washington at the time.

Grassley has said the hearing should take place on Wednesday, but Ford said she wanted it on Thursday at the earliest and to be able to call as a witness a man whom she says was present during the assault.

The committee's Republican leadership turned down those demands.

After several days of maintaining a relatively neutral posture, Trump on Friday declared that Ford could not be believed.

The aggressive stance reflected his fear that time is running out to get his hand-picked conservative judge confirmed - thereby tilting the Supreme Court firmly to the right for years to come - before November mid-term elections when Republicans risk losing control of Congress.

According to Trump, the fact that Ford remained silent until now is an indicator that the incident probably never happened - reasoning that runs counter to what experts say is the typical reaction of sexual assault victims who are too afraid or embarrassed to report.

"I have no doubt that, if the attack on Dr Ford was as bad as she says," Trump wrote, "charges would have been immediately filed with local Law Enforcement Authorities by either her or her loving parents."